Project Summary The goal of this project is to understand how children (and older second-language learners) discover the meanings of words and their semantic roles in sentences. Even for simple words like dog it is not easy to glean what they mean from observing (as one usually will) that there is a dog in sight. For after all, when a dog is in sight, so is his fur, and his ear, and the joyous wagging of his tail. To which of these factors or properties does the pointing finger refer when an adult says Look at that doggie! Things get even harder when the child has to learn the meanings of words like idea or think because in these cases there's nothing so obvious to point to out there in the world. Yet children of three and four years of age understand and utter such apparently abstract words. Our projects take off from the observation that not all words occur in the same places in sentences, for instance one can say I think (or see) that you're cute but not I jump that you're cute. Strikingly, children as young as two- and three-years of age are sensitive to these positional privileges, which in turn give clues to word meaning. Because it is hard to get information about children's word meanings or their learning by asking them for definitions or the like, we use implicit methods such as tracking children's eye gaze direction and responsiveness to queries that place words into different visuo-social environments and into different syntactic structures to find out about their evolving word knowledge. These issues are of much more than academic interest. No tested property of child cognition or behavior is a better predictor of school and work-place success than vocabulary growth in the first few years of life. Vocabulary scores diverge for children of higher or lower socio-economic status as early as the second birthday, and these differences increase throughout the school years, influencing all the child's subsequent learning. So our work extends to discovering ways that actual home and early school environments can maximize the supportive environment for vocabulary and syntax acquisition. Our past work gives strong indications of what these favorable learning environments are, and the present proposal inquires more deeply into these factors.